Mobile holder for a wafer

ABSTRACT

The present invention provides a mobile holder for a wafer, which comprises a base element, a first fixing means and a second fixing means. The first fixing means is configured to allow a wafer to be fixed to the base element. The second fixing means is configured to fix the mobile holder to a support for said mobile holder.

FIELD OF THE INVENTION

The present invention relates to the field of holding devices for wafersand especially to the field of electrostatic holding devices for wafers.

BACKGROUND OF THE INVENTION AND PRIOR ART

In the production of many semiconductor components, the handling of awafer represents an important factor for a successful productionprocess. The production of chip cards and thin integrated circuits, forexample, necessitates that very thin wafers are handled, which have tobe transported and held without the risk of breaking.

Many semiconductor processes, such as the production of circuit elementsfor power electronics, additionally require back-surface processes,which comprise a deposition of solder or alloy layers on the back of thefully processed circuit wafer; in a subsequent alloying or sinteringstep of these layers, high temperatures occur, which may exceed 400° C.The wafer holding device must then guarantee perfect functioning in thecase of these ambient temperatures.

A known method of handling thin wafers is so conceived that the frontface of the wafer is provided with a protective foil, which is typicallya polymer foil. The wafer is held by the carrier foil during thethinning sequence. The possibility of using this known method compriseswafers having a diameter of 6 inches and a thickness which must notexceed 100 μm. Handling and transport of wafers having a thickness ofless than 100 μm is prevented by the fact that the wafers will bend,whereby the risk of breaking increases significantly for the thinwafers. It follows that the method does not offer any possibility ofhandling extremely thin wafers with comparatively large diameters.

Another method makes use of a carrier wafer to which the wafer to bethinned is reversibly attached by means of an adhesive film which isadherent on both sides, e.g. a thermally strippable film. The thermallystrippable film can be stripped by subjecting it to a certaintemperature. The method is adapted to be used for wafers having athickness of not less than 20 μm, and also wafers having a largediameter can be held safely. The method is, however, disadvantageousinsofar as, due to the adhesive film used, which is typically a polymerfilm, the method is not suitable for back-surface processes involvingtemperatures above 130° C.

Furthermore, electrostatic holding devices, so-called electrostaticchucks, are known for taking up wafers by means of electrostaticallygenerated holding forces and for holding these wafers during aproduction process. The holding force can be generated by means of amonopolar electrode or by means of bipolar electrodes. In the case of amonopolar electrode, the wafer to be held serves as a counterelectrodeand must therefore be connected to ground; during a dry etching process,for example, this connection to ground is obtained by the conductiveetch plasma.

The known electrostatic holding devices are, however, disadvantageousinsofar as they have a solid structural design and are, typically,fixedly installed in a processing chamber. In addition, the knownelectrostatic holding devices are dependent on an external power supply.Hence, the known electrostatic holding devices can only be used asholding devices for holding the wafer at one location, but they cannotbe used for the purpose of transport, e.g. for taking up a thinned waferat the location where the thinning sequence has been executed andtransporting it to a second location where e.g. processing of the backsurface will take place.

It follows that the handling of thin wafers, which necessitatehigh-temperature processing steps, such as sintering or alloying, isproblematic, since devices for handling such wafers are not available.

Furthermore, a support device is, at present, not available, whichpermits chips that have been subjected to a dicing process of the typedescribed e.g. in DE-19962763 A1 to be taken up and detachedselectively.

EP 0 552 877 A1 discloses an electrostatic chuck and a method ofexciting the same. The electrostatic chuck is part of a multi-chambersystem for processing integrated circuits. A wafer transport platesupports a wafer by means of an electrostatic holding device. The wafertransport plate itself is connected to a base of the multi-chambersystem by a four-bar connecting joint in such a way that the wafer canbe moved to and fro between various chambers by displacing and rotatingthe wafer support plate. The electrostatic holding device comprises adielectric base layer having arranged thereon electrode strips which areincorporated in a dielectric encapsulating layer. The dielectric baselayer is formed on the upper surface of the wafer support plate makinguse of conventional deposition, masking and etching steps.

EP 0 506 537 A1 discloses an electrostatic chuck comprising an integralfive-layer structure, which is suitable for holding and transporting asemiconductor silicon wafer in a production process for electroniccomponents. A particularly characteristic feature of this electrostaticchuck is that the wafer held will be released immediately when a voltageapplied to electrodes of the chuck is switched off. It is the object ofthe present invention to provide a concept for an improved handling ofwafers.

SUMMARY OF THE INVENTION

In accordance with a first aspect of the invention, this object isachieved by a mobile holder for a wafer, comprising: a base element; afirst fixing means for electrostatically fixing the wafer to the baseelement, said first fixing means comprising a functional layer and anelectrode structure which is arranged between said functional layer andsaid base element, wherein the functional layer is formed such that thefirst fixing means is in an electrostatically active state without anysupply of voltage from outside;and a second fixing means for releasablyfixing the mobile holder to a wafer support.

In accordance with a second aspect of the invention, this object isachieved by a method of handling a wafer comprising the following steps:providing a mobile holder for a wafer, said mobile holder comprising abase element, a first fixing means used for electrostatically fixing thewafer to the base element and including a functional layer which isimplemented such that said first fixing means is in an electrostaticallyactive state without any supply of voltage from outside, and a secondfixing means for releasably attaching the mobile holder to a wafersupport; fixing the mobile holder to a wafer support with the aid of thesecond fixing means; attaching a wafer to the base element with the aidof the first fixing means; moving the mobile holder from a firstlocation to a second location; and detaching the wafer from the mobileholder by actuating the first fixing means.

The present invention is based on the finding that a holder for a wafercan be provided in that a first fixing means is used for fixing a waferto the holder, whereas a second fixing means is used for connecting theholder to a support for the mobile holder.

One advantage of the present invention is that the holder can beattached to a movable support so that the holder is mobile.

Another advantage of the present invention is that various types ofsupports can easily be used for the mobile holder.

A preferred embodiment of the present invention comprises a mobileholder with a flat base element comprising a circular silicon wafer. Onthe front face of the base element an insulating layer is formed whichhas, in turn, formed thereon a first electrode and a second electrode inspaced relationship with one another. The insulating layer consists ofan electrically insulating material, such as silicon oxide. Theelectrodes are provided with respective connecting areas so that theycan be connected to electric leads. The mobile holder is additionallyprovided with a functional layer arranged over the full area of thefirst electrode and of the second electrode and consisting of a materialwhich contains movable ions. In the preferred embodiment, the functionallayer allows the mobile holder to be in an active state without beingconnected to an external voltage source. On top of the insulating layera cover consisting of an insulating material, such as silicon oxide,silicon nitride or silicon carbide, is formed as a protective layer overthe full area of the insulating layer.

In an alternative embodiment, the mobile holder comprises an activelayer consisting of a material having a high dielectric constant.

In a further embodiment, the mobile holder has a matrix electrodestructure in which individual electrodes can be controlled purposefullyand supplied with voltages so as to permit a pixelwise removal of chips.

Furthermore, another embodiment is provided with feed-through leads forthe first electrode and the second electrode, which extend through thebase element so as to permit a connection of the electrodes from theback of the base element.

In one embodiment, the second fixing means constitutes the area of theback of the mobile holder. In this embodiment, the support for themobile holder is a conventional vacuum wafer chuck, the mobile holderbeing attached to the vacuum wafer chuck by a vacuum applied between thesecond fixing means and a fixing area of the wafer chuck.

In a further embodiment, the second fixing means is a margin of themobile holder. In this embodiment, an attachment of the mobile holder toa known support is achieved in that the support mechanically grips themobile holder on the margin thereof.

Furthermore, in the case of still another embodiment, the edge of themobile holder constitutes the second fixing means, a known supportfixing the mobile holder by mechanically gripping the mobile holder atthe edge thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, preferred embodiments of the present invention will beexplained in detail making reference to the drawings enclosed, in which

FIG. 1 shows a cross-sectional view of a mobile holder according to oneembodiment of the present invention; and

FIG. 2 shows a top view of a mobile holder according to one embodimentof the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

According to FIG. 1, a mobile holder comprises a flat, circular baseelement 1 having an insulating layer 2 formed on the front face thereof.The insulating layer 2 consists of an electrically insulating materialsuch as silicon oxide or silicon nitride. A first electrode 3 and asecond electrode 4 are arranged on the insulating layer 2 in spacedrelationship with one another. The first electrode 3 and the secondelectrode 4 preferably consist of a temperature-resistant electricconductor, such as a metal having a high melting temperature, and areidentical in area, as will be explained in the following with referenceto FIG. 2.

The base element 1, which preferably consists of a semiconductormaterial, ceramics such as ALO₂, Kapton or SiC, has a diameter which isequal to the diameter of a wafer to be held or which is slightly largerthan the wafer to be held. In, a specially preferred embodiment, thebase element 1 is formed such that it has a diameter of approx. 150 mmand a thickness of approx. 680 μm; this corresponds to typicaldimensions of a standard wafer.

Due to the use of temperature-resistant materials for the base element1, the mobile holder is suitable for high ambient temperatures so thatthe mobile holder is suitable for holding process wafers duringtemperature-critical processes, such as an alloying or sintering ofmetal layers.

Silicon and other semiconductor materials show at high temperatures anincrease in their intrinsic conductivity, which causes a deteriorationof the electrostatic holding force in the case of the voltages appliedto the electrodes, which are in the range of from 1,000 V to 2,000 V. Inprocesses involving high temperatures, e.g. in plasma processes,ceramics and in particular SiC are preferred to semiconductor materialsas a material for the base element 1; due to the high thermalconductivity, SiC additionally guarantees that the wafer to be held iswell cooled in an advantageous manner. SiC has the further advantagethat the expansion coefficient does not essentially differ from that ofsilicon so that, especially in cases in which the wafer to be heldconsists of silicon, a use of SiC as a material for the base element 1will be advantageous.

The mobile holder additionally includes a functional layer 5 which isarranged on top of the first electrode 3 and the second electrode 4 andin the spaces between these electrodes and which extends over the wholesurface of the wafer as a continuous layer. In the preferred embodimentthe functional layer 5 consists of a material containing movable ions,such as borosilicate glass, or of a material having a high dielectricconstant, such as barium titanate, strontium titanate.

On top of the electrically insulating layer 5 a cover 6 consisting of aninsulating material, such as silicon oxide or silicon nitride, is formedover the full area of the insulating layer 5. The cover serves toprotect the functional layer 5 arranged therebelow against aggressivechemical substances.

FIG. 2 shows a top view of the mobile holder shown in FIG. 1, FIG. 1 andFIG. 2 being not represented on the same scale. As can be seen in FIG.2, the first electrode 3 has a first connecting area 7 and the secondelectrode 4 has a second connecting area 8, each of these connectingareas being arranged in the vicinity of the edge of the mobile holder.The connecting areas 7 and 8 permit an electric voltage to be appliedthe respective associated electrodes. The arrangement of the connectingareas in the vicinity of the edge of the mobile holder allows an exposedmode of arrangement of these connecting areas when a process wafer isattached, in that said process wafer is attached to the mobile holder insuch a way that the flat portion of the wafer is located in the regionof the connecting areas. The term flat portion describes a portion inwhich the edge of the wafer, instead of following a circular-arc shape,follows a straight line of a segment portion so as to indicate thedirection of crystal growth. It follows that, in this embodiment,electric leads for the connecting areas can be fixed to the front faceof the mobile holder.

In an alternative embodiment, feed-through leads for the first electrode3 and the second electrode 4 extend through the base element 1 so that,by means of connecting areas formed on the back of the base element 1,the electrodes can be electrically connected to leads. Leads extendingthrough the base element have the advantage that the process wafer canbe positioning independently of the position of the flat.

In the embodiment shown in FIG. 2, the first electrode 3 and the secondelectrode 4 are, in addition, arranged in such a way that the electrode3 is essentially arranged in a first quarter circular segment of thecircle area of the base element 1 and in a second quarter circularsegment located in diagonally opposed relationship with the first one,the electrode portions arranged in these quarter circular segments beinginterconnected by a bridge which extends through the centre of thecircle area of the base element 1. Furthermore, the electrode 4according to FIG. 2 is essentially formed in the quarter circularsegments in which the electrode 3 is not arranged. The two diagonallyopposed quarter circular segment portions of the electrode 4 areinterconnected via an elongate, curved connection area extending alongthe edge of the base element 1.

The first electrode 3 and the second electrode 4 are arranged in such away that they are spaced apart by an elongate interspace having anessentially constant width. Furthermore, the electrodes 3 and 4 arespaced from the edges of the base element 1 in such a way that a marginis formed between the electrodes 3 and 4 and the edge of the baseelement 1, said margin being adapted to be used as a holding device forattaching the mobile holder to a support.

For attaching a wafer to be held, e.g. a very thin process wafer (20–100μm), the front face of the mobile holder must be brought into contactwith or into close proximity to the side of the wafer to be held atwhich the holder is to be attached.

In order to move the mobile holder to the take-up location, said mobileholder is first fixed to a wafer support, e.g. a robot wafer support. Ina preferred embodiment, the margin formed between the electrodes and theedge of the wafer serves as a fixing means for fixing the wafer support;in this embodiment, the mobile holder has a slightly larger diameterthan the wafer to be supported so that the process wafer to be taken updoes not project into the marginal area of the mobile holder and can befixed without any disturbance by an attachment means fastened to themargin of the mobile holder. The mobile holder can be fixed to anattachment means of the wafer support through pressure contact or insome other suitable way, such as reversible bonding.

In a further embodiment, the edge of the base element 1 represents thefixing means for attaching the mobile holder to a wafer support. In thisembodiment, the mobile holder is only held by pressure contact betweenan attachment means of the wafer support and the edge of the mobileholder.

Furthermore, in another embodiment the back of the mobile holder mayhave the function of a fixing means for attaching a wafer support.Alternatively, the mobile holder can also be transferred to a stationarychuck, such as an electrostatic chuck, said chuck being secured to theback of the mobile holder.

For facilitating a transfer of the mobile holder, said mobile holder mayadditionally be provided with suitable adjustment and sensor elements.

As has already been mentioned, the shape and the size of the mobileholder are preferably of such a nature that said mobile holder differsonly insignificantly from a standardized shape and size for processwafers; even wafers having a large diameter and a small thickness can beheld easily by the mobile holder. The standardized shape of the mobileholder makes it possible that the mobile holder can be stored instandardized storage devices for wafers, e.g. in racks or hurdles, andthat it can be taken up from said racks by standard wafer supports, suchas robot handling devices, and transported to desired locations. In viewof the fact that it is possible to use the holding and gripping devicesprovided as part of the standard equipment in the field of semiconductorproduction, the mobile holder according to the present inventionrepresents a reasonably-priced system, which, in addition, can easily beattached to an arbitrary type of wafer support, a circumstance whichessentially facilitates integration in a semiconductor productionprocess.

When the mobile holder has been fixed to the wafer support, the mobileholder can be moved by means of said wafer support to the process waferto be held.

In order to make it possible that the wafer to be held is held on themobile holder, the mobile holder must be in an activated state in whichthe electrodes 3 and 4 have applied thereto opposite voltages which aresufficiently high for fixing a wafer. Due to the provision of thefunctional layer 5, the mobile holder is implemented such that it willmaintain an activated state also without any external voltage source.This is achieved by the use of suitable materials for the functionallayer 5. The material employed so as to be able to use the functionallayer 5 as a storage layer for storing electrostatic charges in anactivated state is preferably a dielectric material with movable ions,such as borosilicate glass, which contains movable sodium and potassiumions. Alternatively, a material having a high relative dielectricconstant can be used.

The use of the above-mentioned materials allows a quasi-permanent chargeshifting according to the known capacitor effect. The mobile holder isonly connected to an external voltage source for the purpose of chargingand is in an activated state also after having been separated from theexternal voltage source; in this activated state, a voltage which issufficiently high for holding the process wafer is applied between thefirst electrode 3 and the second electrode 4. In the case of prolongedholding times, the mobile holder can be connected via the connectingareas 7, 8 to the external voltage source for the purpose of“refreshing” the charges.

In the activated state, an electric field is produced by the oppositelycharged electrodes 3 and 4, said electric field causing in a wafer to besupported, which is in contact with or in close proximity to the frontface of the mobile holder, a charge transfer, whereby a force isproduced which holds the wafer to be held on the front face of themobile holder. Depending on the respective field configuration, positiveor negative charges are generated on the surface of the wafer to besupported. It follows that a wafer held on the mobile holder has asurface charge distribution comprising essentially areas with positiveand negative charges which correspond to the arrangement of theelectrodes 3 and 4. In other words, an electrode 3 produces e.g. apositive charge on the surface of a wafer to be held in the area locatedopposite to said electrode 3, whereas the electrode 4 produces anegative charge on the surface of a wafer to be held in the area locatedopposite to said electrode 4. It follows that the arrangement of theelectrodes 3 and 4 in segments of a circle area has the advantage that,by connecting an external voltage source, it is possible to reverse thepolarity of or to disable one of the two electrodes so that a holdingforce can be reduced and switched off.

The wafer holder of above-described embodiment can be used in aparticularly advantageous manner for dicing chips in connection with dryetching. This kind of use will be explained in the following.

The process wafer is here first prestructured with trenches for dicingthe chips on the front face of the process wafer. In order to permit aremoval of diced chips, the trench structure must correspond to thearrangement of the electrodes. In other words, the above-describedembodiment of a mobile holder with an electrode arrangement in quartercircular segments can be used for dicing a wafer intoquarter-circular-segment chips, the structured trenches subdividing thewafer into quarter circular segments.

The depth of the structured trenches is chosen in accordance with thedesired residual thickness of the wafer, which the wafer is intended tohave after a thinning and dicing process; in the case of e.g. thin filmchips, this residual thickness is in the range of from 40 to 80 μm.

For producing thin chips, the thinning process is first carried out inthe usual way, e.g. by grinding and stress-relief etching, until athickness of approx. 100 μm has been reached. The prestructured trenchesare still closed at that time. Subsequently, the wafer is subjected to athinning process executed e.g. in a vacuum plasma chamber. The wafer canbe transported into the plasma chamber either with the mobile holder orwith a known holding device and in said plasma chamber it can be fixedto a stationary holding device, such as an electrostatic chuck.

Alternatively, the wafer may also remain on the mobile holder forcarrying out the thinning in the vacuum plasma chamber.

In the plasma chamber, the wafer is then thinned by dry etching the backuntil the desired thickness of approx. 40 to 80 μm has bee reached,whereby the trenches are opened from the back. By opening the trenches,the wafer will automatically be diced into individual chips. Followingthis, the mobile holder, which can be kept e.g. in a rack in the vacuumplasma chamber, is fixed to a movable support, e.g. a robot wafersupport. The fixing to a suitable support can be carried out easily,since the mobile holder has essentially the shape and the size of awafer, the support seizing the holder e.g. at the edge or the margin ofthe holder.

Subsequently, the front face of the mobile holder is moved into closeproximity to the diced chips so that the segment portions of the firstelectrode 3 and of the second electrode 4 are located in opposedrelationship with the respective chips. For effecting a precisealignment a suitable adjusting device can be used. Furthermore, sensorscan be provided on the mobile support so as to allow exact alignmentwith respect to the chips.

Following this, the stationary electrostatic chuck releases the dicedchips, which are then taken up by the mobile holder. The mobile holderis then transported to a discharge location by the movable support. Atthe discharge location, the diced chips attached to the mobile holderare selectively removed from said mobile holder by reversing thepolarity of electrode 3 or electrode 4.

The polarity of the electrodes is reversed by applying apolarity-reversal voltage with opposite polarity to the connecting areaof the respective electrode. If, for example, the polarity of electrode3 is reversed, the surface charge in chips located opposite to saidelectrode 3 will be diminished, whereby the holding force will bereduced and the respective chips will be detached from the mobile holderwhen a lower limit of the holding force has been reached.

The wafer having a thickness of approx. 100 μm is then fixed to themobile holder in such a way that the quarter circular segments of thewafer, which correspond to the chips to be diced, are located in opposedrelationship with the quarter circular segments of the electrodes 3 and4 of the mobile holder.

The described embodiment of the present invention provides thepossibility of selectively detaching two of the fourquarter-circular-segment chips by reversing the polarity of one of thetwo electrodes. In order to permit a selective detachment of each of thefour quarter-circular-segment chips, an electrode structure of fourseparate quarter-circular-segment electrodes is provided in the case ofa further embodiment, each of these quarter-circular-segment electrodesbeing provided with a connecting area for applying a voltage. First, theelectrodes are activated by switching the leads in a suitable mannerduring the activation according to the embodiment of FIG. 2, so that tworespective diagonally opposed electrodes will have the same polarity.When the chips have been taken up, one or a plurality of thequarter-circular-segment chips can be detached selectively bypurposefully reversing the polarity of the four quarter-circular-segmentelectrodes, whereas the rest of the quarter-circular-segment chips isstill held by the mobile holder. The concept of independent electrodes,which define in the above-described embodiment an electrode structure inthe form of a 2×2 matrix, can, in a further embodiment, be extended to alarger number of electrodes which are arranged in a matrix having mcolumns and n lines.

By providing the electrode structure in the form of a matrix, individualchips can be removed “pixelwise” by reversing the polarity of respectiveelectrodes of the matrix. In this embodiment, the mobile holder can beused for dicing a plurality of chips from a wafer.

Furthermore, the mobile holder according to the present invention can beused for selectively taking up diced chips from a plurality of chips bycontrolling the connection voltages in a suitable manner.

1. A mobile holder for a wafer, comprising: a base element; a firstfixing means for electrostatically fixing the wafer to the base element,said first fixing means comprising: a functional layer including acharge storage layer; an insulating cover arranged on top of saidfunctional layer, said wafer being in contact with said insulatingcover, when said wafer is fixed to said mobile holder; and an electrodestructure comprising a first electrode and a second electrode, eachelectrode having a connecting area for connecting said electrodestructure to electric leads, wherein the first and the second electrodesare arranged on top of the base element and below the functional layer,wherein the functional layer is formed such that the first fixing meansis in an electrostatically active state without any supply of voltagefrom outside after applying a voltage to said connecting areas, saidelectrostatically active state being such that a wafer is held at saidmobile holder due to charges stored in said charge storage layer withoutany supply of voltage from outside; and a second fixing means forreleasably fixing the mobile holder to a wafer support.
 2. A mobileholder according to claim 1, wherein the electrode structure comprisesmore than two electrodes.
 3. A mobile holder according to claim 2,wherein the more than two electrodes are arranged in a matrix.
 4. Amobile holder according to claim 1, wherein the connecting areas arefixed to an area of said first fixing means that corresponds to a flatsection of a wafer held by said mobile holder.
 5. A mobile holderaccording to claim 4, wherein the base element has a front face and aback, the front face having arranged thereon the first fixing means, andthe front face of said base element having arranged thereon saidconnecting area.
 6. A mobile holder according to claim 1, wherein thebase element has a front face and a back, the front face having arrangedthereon, the first fixing means, and the back of said base elementhaving arranged thereon said connecting area, said base elementcomprising a feed-through lead for connecting the connecting area to theelectrode structure.
 7. A mobile holder according to claim 1, whereinthe functional layer consists of borosilicate glass, barium titanate orstrontium titanate.
 8. A mobile holder according to claim 1, wherein thebase element consists of a semi-conductor material, or ceramics.
 9. Amobile holder according to claim 1, wherein the base element has a frontface and a back, the front face having arranged thereon the first fixingmeans and the back of the base element being implemented as an area andrepresenting the second fixing means so that the mobile holder isadapted to be releasably connected to a vacuum chuck as a wafer support.10. A mobile holder according to claim 1, which comprises as a secondfixing means a marginal area into which a wafer held by the first fixingmeans does not project, said marginal area being implemented for fixingto the wafer support.
 11. A mobile holder according to claim 1, whereinthe base element comprises as a second fixing means an edge which isimplemented such that the mobile holder is adapted to be releasablyfixed by the wafer support simply by means of pressure contact betweenthe wafer support and said edge.
 12. A mobile holder according to claim1, which comprises a front face and a back, the front face being adaptedto have the wafer attached thereto and the back being implemented as asecond fixing means for fixing to an electrostatic chuck as a wafersupport.
 13. A mobile holder according to claim 1, which has a shape ofsuch a nature that it is adapted to be stored in wafer storage devices.14. A method of handling a wafer comprising the following steps:providing a mobile holder for a wafer, said mobile holder comprising abase element, a first fixing means used for electrostatically fixing thewafer to the base element and a second fixing means for releasablyattaching the mobile holder to a wafer support, said first fixing meanscomprising: a functional layer including a charge storage layer; aninsulating cover arranged on top of said functional layer, said waferbeing in contact with said insulating cover, when said wafer is fixed tosaid mobile holder; and an electrode structure comprising a firstelectrode and a second electrode, each electrode having a connectingarea for connecting said electrode structure to electric leads, whereinthe first and the second electrodes are arranged on top of the baseelement and below the functional layer, wherein the functional layer isformed such that the first fixing means is in an electrostaticallyactive state without any supply of voltage from outside after applying avoltage to said connecting areas, said electrostatically active statebeing such that a wafer is held at said mobile holder due to chargesstored in said charge storage layer without any supply of voltage fromoutside; fixing the mobile holder to a wafer support with the aid of thesecond fixing means; attaching a wafer to the base element with the aidof the first fixing means; moving the mobile holder from a firstlocation to a second location; and detaching the wafer from the mobileholder by actuating the first fixing means.